1. Field of the Invention
The present invention relates to a color signal processing circuit for a VCR (video cassette recorder), and more particularly, to a color signal processing circuit which is in the form of an IC (integrated circuit).
2. Description of the Prior Art
Of color signal processing circuits, for a VCR, which is in the form of an IC, a band-pass filter of 3.58 MHz is used for both recording and playback (reproduction) systems. When a signal is inputted to the band-pass filter, an input signal is changed over by a selection switch (recording/playback changeover switch). Moreover, in the color signal processing circuit for a VCR, an input color burst is changed over by a selection switch when a color signal for recording and that for playback are provided to each automatic phase control (APC) circuit.
In FIG. 1, solid lines show a recording system, whereas alternate long and short dash lines show a playback system. Blocks 3 and 4 are used for both recording and playback. In the recording system, a color signal inputted from an input terminal 1 is provided to a gain variable amplifier 3 after passing through a band-pass filter 2 with a center frequency of 3.58 MHz via a switch 7. An output of the gain variable amplifier 3 is decreased in frequency to 629 kHz by a balanced modulation circuit 4, and thereafter, is directed to an output terminal 6 via a low pass filter 5.
In the playback system, a playback color signal inputted into an input terminal 9 is provided to the gain variable amplifier 3 after passing though a band-pass filter 10 of 629 kHz. In this case, an output of the gain variable amplifier 3 is converted into a frequency witha carier of 3.58 MHz at the balanced modulation circuit 4 on the next stage. After passing through the selection switch 7, a band-pass filter 2 of 3.58 MHz, and a comb filter 11, it is amplified by an amplifier 12, and thereafter, is directed to an output terminal 13. The color signal directed to the output terminal 13 is further directed to a Y/C mixer (not shown) connected to the output terminal 13 to be combined with a luminance signal.
In FIG. 1, the selection switch 7 is set to a contact R at the time of recording to direct a color signal from the input terminal 1 to the band-pass filter 2. At the time of playback, the selection switch 7 is changed over to a contact P to direct a color signal from the balanced modulation circuit 4 to the band-pass filter 2.
With respect to the APC, a color signal is supplied from the gain variable amplifier 3 to a REC APC circuit 14 for recording through a selection switch 8 at the time of recording. At the time of playback, a color signal is supplied from the amplifier 12 to a PB APC circuit 15 through a selection switch 8. The REC APC circuit 14 is a circuit for locking a color burst (of 3.58 MHz) of an input color signal and a variable oscillation frequency of a crystal oscillator to generate a stable signal of 629 kHz. The PB APC circuit 15 is a circuit for locking a color burst (of 3.58 MHz) of a color signal from the amplifier 12 and a fixed oscillation frequency of the crystal oscillator by varying an oscillation frequency of another voltage controlled oscillator to generate a stable signal of 3.58 MHz. Since the REC APC circuit 14 is designed to operate only at the time of recording and the PB APC circuit 15 is designed to operate only at the time of playback, no problems arise even if an output of the selection switch 8 is simultaneously provided to the two APC circuits as shown in the figure.
Now, a problem will be described with reference to FIG. 2 showing a main protion, relating to the selection switch 8, of FIG. 1. In FIG. 2, at the time of recording when the selection switch 8 is changed over to the contact R, a color signal from the gain variable amplifier 3 is supplied to the REC APC circuit 14 and the PB APC circuit 15 through the selection switch 8, whereas an output of the amplifier 12 is shut off by the selection switch 8 so as not to be coupled to the REC APC circuit 14. Moreover, at the time of playback when the selection switch is changed over to the contact P, a color signal from the amplifier 12 is supplied to the PB APC circuit 15 through the selection switch 8, and an output of the gain variable amplifier 3 is shut off by the selection switch 8 so as not to be coupled to the REC APC circuit 14 and the PB APC circuit 15.
Since an output signal line 16 of the gain variable amplifier 3 and an output signal line 17 of the amplifier 12 run long in an IC until they reach a switch block 18, problems arise that an un-selected signal line affects the other signal line and that it affects an adjacent third circuit on the way to the switch block 18. These problems are remarkable particularly in a large scale integrated circuit where signal lines are densely arranged.
To solve these problems, a method is sometimes adopted where a switch is provided to each signal block (in FIG. 2, to the gain variable amplifier 3 and the amplifier 12) so that signals are not outputted to the output signal lines of the switch blocks when a signal thereof is not used. In that case, not only each block is increased in size but also another control line is required for controlling the switch provided to each signal line. Consequently, the number of lines increases as well as degree of integration decreases, whereby the arrangement of the IC is complicated. The same problems arise with respect to a portion of the circuit relating to the selection switch 7.